Multi‐Elevation UAV‐Based Frequency Domain Electromagnetic Method for Data Calibration and Field Investigation

The Frequency Domain Electromagnetic Induction method (FDEM) is an efficient tool for investigating the electrical conductivity (EC) distribution over relatively shallow depths. However, the handheld method of use recommended by manufacturers does not fully leverage the non‐invasive detection capabilities offered by FDEM devices. In this study, an unmanned aerial vehicle airborne FDEM (UAV‐FDEM) system is introduced, which enables an operator to conduct investigations at specific flight heights along planned routes. Since multi‐coil FDEM instruments typically experience consistency issues among different coils, we propose a calibration method based on a multi‐elevation UAV‐FDEM approach. The approach circumvents the need for geophysical inversion during calibration, and has been successfully employed to calibrate two multi‐coil instruments. We tested the multi‐elevation UAV‐FDEM survey approach at two sites: a riparian zone of Yangtze River and a hot spring area in Tibet. The results show that the UAV‐FDEM survey findings are comparable with those obtained using electrical resistivity tomography (ERT). The surveys detected temporal changes in soil EC that correspond with observed groundwater levels changes, and successfully delineated the intrusion area and subsurface path of geothermal water. In comparison to conventional ground‐based single‐elevation measurements, the multi‐elevation UAV‐FDEM method clearly improves the deterministic coefficients (that is measures of resolution) for the inverted EC value of different soil layers, and reduces the uncertainty of the geophysical inversion results. UAV‐based FDEM surveys are efficient for large or inaccessible areas, but their application can be limited by adverse weather and restricted flight endurance.